Genetic recombination and conditional knockouts

Certain viral enzymes, such as Cre recombinase, work to integrate viral DNA into the genome of their host. Researchers can use this enzyme to engineer tissue-specific knockout mice. This is an important technology because deleting a gene from an organism completely can be lethal if that gene influences vital, ubiquitous processes. Even if a total body knockout does not yield a lethal phenotype, a tissue-specific, or “conditional,” knockout mouse can still be helpful by enabling investigators to analyze the effect of the gene’s absence in a specific lineage of cells. For example, one might investigate the effect of knocking out Caveolin-1 (Cav-1) in macrophages, as this ubiquitously-expressed gene codes for a protein that is important for endocytosis and phagocytosis, but does not result in a lethal phenotype when knocked out in the entire body (denoted as Cav-1–/–). Cre recombinase specifically cleaves DNA at consensus sequences called loxP sites. If researchers synthesize a gene flanked by loxP sites (this would be called a “floxed” locus), that locus is susceptible to elimination in the presence of Cre recombinase (Figure 1).

Figure 1. Cre-Lox Conditional Knockout Technology

A key step in generating a conditional knockout is selecting a promoter to drive expression of the inserted Cre gene. For example, myeloid cells (including monocytes, macrophages, and neutrophils) express LysM, the gene for lysozyme M. Using the LysM promoter, one can express Cre recombinase in these myeloid cells only (Figure 2).

Figure 2. LysM promoter specifies Cre expression (CDS = coding sequence)